During the machining of cylindrical workpieces on lathes collets are often used to clamp the workpieces in place. This is true to an even greater extent for the machining of rod material on semi- or fully automatic lathes of single or multiple spidle design. In a large number of such lathes the collets are tightened by means of a tensioning device. On conventional collets suitable for this purpose and made in a single piece, a clamping head part and a clamping tube part may be distinguished. The clamping tube part has on its inner side clamping surfaces for the workpieces and on its outer side an external cone located at least in part in the longitudinal section of the clamping surfaces and diverging in the direction of the free front side facing away from the clamping tube part. The clamping head part is divided into individual gripping jaws by longitudinal slits arranged in a distribution over the circumference and continuous in the radial direction. These longitudinal slits originate at the free front side of the clamping head part and extend in the axial direction through the section of length with the clamping surfaces and the external cone forming the clamping head part proper, through an intermediate section of length which in its original state is of a tubular shape, to a section of length remaining unslit and forming the clamping tube part proper. At the end of the clamping tube part facing away from the clamping head part, a fastening screw thread is provided for the fastening of the collet to the tensioning device of the lathe. The clamping head part in most cases additionally has a circular cylindrical guide surface on its outer side, the external diameter of which is adapted to the internal diameter prevailing in this section of length of the main spindle, in order to guide the rear end of the collet in the radial direction. The intermediate section of length located between the clamping head part proper and the clamping tube part proper, which as a rule has an external diameter smaller than that of the clamping tube part or at least than that of its guide surface, is divided by the longitudinal slits into individual tongues by means of which the gripping jaws of the clamping head part, connected in front with said tongues, may move elastically in and out in the radial direction with respect to the clamping tube part. The tongues are spread to a certain degree so that in the unclamped condition of the collet the internal opening width of the gripping jaws is larger than the nominal diameter, permitting the insertion of the workpieces between the gripping jaws without difficulty.
The hollow main spindle of a lathe has at its front end an internal cone, to which the outer cone of the collets is adapted. The angle of cone of the external cone is usually larger by 10 to 30 angular minutes than that of the inner cone, the external cone thus being somewhat less acute than the inner cone, in order to prevent the jamming of the gripping jaw in the cone. The tensioning device of a lathe generally has a draw tube, which extends from the rear end of the main spindle, into said spindle, to the clamping tube part of the collets. The draw tube is equipped at its front end with a fastening screw thread to which the fastening screw thread on the clamping tube part of the collets is adapted. For use in a lathe, the collet is tightly screwed together with the draw tube and adjusted together with said draw tube with respect to the tensioning device in the axial direction, so that in the release position of the tensioning device the external cone surfaces of the gripping jaws, because of the spread of the tongues, are already in contact with the internal cone of the main spindle, but their internal width is still larger than the nominal diameter and that in the locking position of the tensioning device the internal width of the gripping jaws is reduced to a measure less than the external diameter of the workpieces, while taking into consideration the elastic deformation of the parts.
During the locking or clamping motion the clamping jaws mounted on the front end of the tongues move along an arc and in the process perform a swivel motion. If the workpiece does not exactly conform to its nominal diameter but is over- or undersized, it will be gripped only by the front or rear ends of the gripping surfaces and thus clamped over a relatively small section of its length. The moment generated by this condition at the gripping jaws in general is not capable of deforming the tongues elastically strong enough so that the gripping surfaces of the gripping jaws would contact the workpiece over a larger section of length or possibly over their entire length. Because the tools engage the workpiece outside the clamping location, the danger of the workpiece giving way under the cutting forces increases with the magnitude of the difference between the over-or undersize of the workpiece in relation to the nominal diameter of the collet. Tolerances, particularly tolerances with respect to the diameter, of the workpiece being machined increase to the same degree, together with the danger of the formation of chatter marks on the surface of the workpiece. All of this makes it necessary to space the collets of a set of collets very closely together, leading to a great number of collets and accordingly to high costs of the set. In addition, these conditions also force the observation of low diametrical tolerances of the workpieces and for example in the processing of bar material make it necessary to forego the use of the less expensive as-rolled material in favor of the more expensive drawn or peeled bars. In view of the high stresses to which the gripping jaws are exposed both in the area of the gripping surfaces and of the external cone, where high wear resistance is desired, and also in the area of the spring tongues in the section of length between the clamping head part and the clamping tube part, where high tensile strength and high elasticity are desired, conventional collets are made of a high strength material, generally of a tool steel, which however, is capable of satisfying the various requirements subject to different restrictions only. During the fabrication of the hollow collets, which in most cases are thin-walled, a large part of the material volume of the blank must be machined away. Conventional collets are therefore relatively expensive both with respect to material usage and fabrication costs.
During a change of collets, conventional collets must be drawn out with their entire length from the main spindle, following their unscrewing from the draw tube of the tensioning device. This operation is often impeded by carriages or other devices of the lathe, located in opposing positions and cannot always be facilitated by additional adjustments of such devices.